If you are looking for an advanced roller coster designer & simulator, No Limits is hands down, the best.

The program is divided into a wire-frame editor and a fully rendered 3D sim and you can easily jump back and forth between the two.

You can create and virtually any type of coaster from the classic woodies, to Arrow loopers, to the cutting edge coasters from B&M and Intiman.

No Limits makes it easy to add your own custom textures to your coaster trains and if you have access/the knowledge of a program like 3D Studio Max, you can create your own .3ds objects to import right into your coaster.

I wouldn't really compare it to RCT3 as its not a park simulator. No Limits is strictly coasters. No park guests, no money, no flat rides, no food. Just coasters and all the physics you can stand.

There really is no competition for this program. There was, but they have gone unmaintained for a while, and they are only for PC.